The present patent application concerns a method for cooling. The cooling is achieved by a pressure-induced phase transition whereby the crystal field interaction is altered.
Different technical approaches are employed in science and technology for cooling samples and other objects. Depending on the area of application the below physical effectsxe2x80x94beneath othersxe2x80x94are being made use of:
evaporation of liquids on surfaces;
Peltier-effect;
adiabatic expansion of gas; and
adiabatic demagnetization;
Details concerning the adiabatic demagnetization are for example given on pages 472 and following of the German book with title xe2x80x9cEinfxc3xchrung in die Festkxc3x6rperphysikxe2x80x9d, by Ch. Kittel, 8th revised edition, published by R. Oldenburg Verlag GmbH, Germany. The adiabatic demagnetization is also addressed in Ch. Kittel""s book xe2x80x9cIntroduction to Solid State Physicsxe2x80x9d, 3rd edition, Wiley, New York, London, Sidney, 1967, chapter 14, p 440 and FIG. 8. The other effects/techniques are well known and addressed in-depth in the appropriate literature, too.
Liquid gases are employed for cooling objects as well. The mentioned physical effects and the respective methods for cooling and the apparatus for cooling which are based thereon are either limited to a narrow temperature range, or the associated technical effort is rather big. The associated technical effort may lead to increased costs of the method for cooling and the corresponding apparatus. Furthermore, a big effort is required to gain control of inherent security risks.
Explanations of these known cooling methods and apparatus can be found in the appropriate technical literature. The technical literature is not listed herein since the following invention deviates to a great extent from any of the known approaches and a detailed discussion of the known methods is not relevant for the understanding of the present invention.
S. G. Rosenkranz describes in his German Ph.D thesis with title xe2x80x9cNeutronendiffraktion und Neutronenspektroskopie an Seltenen Erd-Nickelaten RNiO3 (R=Seltene Erde)xe2x80x9d, ETH Zurich, Ph.D. thesis ETH No. 11853, 1996, Switzerland, that certain rare earth nickelates undergo a structural change if an external pressure is applied. This Ph.D thesis mainly concerns the experimental investigation and theoretical discussion of the temperature dependence of the phase transition (change of the symmetry of the crystal field) and structural changes of crystals.
Structural changes of so-called Laves-phases were described for the first time by O. E. Martin and K. Girgis in J. Magn. Magn. Mater., Vol. 37, p. 228-230, 1983.
It is an object of the invention to develop a simple method for the cooling of objects and an apparatus being based thereon.
The method and the apparatus based thereon shall be inherently secure and its realization shall be possible with acceptable effort and at low cost.
The objects of the invention have been accomplished by a method and apparatus making use of special materials, the crystal symmetry of which changes if pressure is applied or reduced such that a structural phase transition occurs where the material""s crystal field interaction is altered. The crystal field interaction might either be altered by a transition from a state with strong degeneracy of the crystal field to a state with reduced degeneracy of the crystal field, or it might be altered such that the overall crystal field splitting is changed. The special material cools down during this transition. The cooling effect can be employed to cool objects which are thermally coupled to the special material.
In principle, all materials are suited for use in the present context which comprise ions or atoms which show at a certain crystal symmetry a degenerate crystal field of their deepest levels and which undergo a pressure-induced phase transition towards a state with reduced or removed degeneracy of the crystal field. This phase transition is pressure induced and can be caused by an isotropic and/or uniaxial pressure application or depressurization. Well suited materials are rare earth compounds such as rare earth nickelates, rare earth manganates, rare earth aluminates, for example, as well as other transition metal oxides which show a phase transition if pressure is increased or reduced and which as a result cool down. Also suited are materials which undergo a pressure-induced phase transition whereby the degeneracy of the crystal field is not reduced or removed, but the overall crystal field splitting is changed. Alloys of a rare earth (R) and metals, such as Aluminum (Al) and Gallium (Ga), for example, are materials which show such a behavior. The so-called xe2x80x9cLaves phasesxe2x80x9d RAl2xe2x88x92xGax (R=Nd, Er; 0xe2x89xa6xc3x97xe2x89xa62) are an example of a well suited material.
These effects were not known until now and have not been used technically despite the fact that rare earth nickelates had been made and analyzed for the first time more than twenty years ago. S. G. Rosenkranz has observed in connection with the above-mentioned Ph.D thesis for the first time that certain rare earth nickelates undergo a structural change if an external pressure is applied. The structural changes of Laves phases have been described by O. E. Martin and K. Girgis in the above-mentioned publication in 1983. The present invention is based on the work by S. G. Rosenkranz as well as by O. E. Martin and K. Girgis.
The invention is furthermore based on the observation that a controlled phase transition occurs if a pressure is applied, or if the pressure is reduced (depressurization). In addition, the present invention makes use of the fact that the entropy changes with the phase transition and that a controllable cooling can thus be achieved.
It is an advantage of the method for cooling and the apparatus based thereon, as claimed, that it can be used in a temperature range between 0 less than Txe2x89xa6600 Kelvin (xe2x88x92273 less than Txe2x89xa6327 degree centigrade).
It is a further advantage that the inventive method for cooling can be optimized across the entire temperature range by modifying the chemical composition of the materials utilized.